So 2002 merc mag mpi with 555. Ran last weekend for 5-6 hours pretty hard. Went all day with 2k lbs and a few people no issue. On the way in I came out of the no wake under a bridge hard to avoid some rollers and hit into guardian mode. For those who don't know guardian is basically the computer nazi that analyzes everything going on and tries to protect the system....

Anyways so I hit 3540 rpm and goes into protection. After that any time I went hard on the gas it just bogged and went into protection.

We threw it on the computer yesterday morning and sea water level was showing as low in the computer. Basically tore the entire cooling system apart up to the sensor. Had some slight debri in the screen right before the sea water sensor. Somehow it had made it past the check valve.

Impeller was in perfect shape, pump was rotating freely, and no grooves inside.

Cleaned that out and headed to the lake. Now it only does it maybe half the time and thats if you reallly get into it. Was able to get a set in just going 75% power up to 3200 or so.

Now heres the odd part..... If I take it slow at maybe 50% throttle and get it above 3000rpm I can slam it all day long. Was running 40-42mph on the way in no issue and well into the 4000's on rpm.

Any ideas? Nothing else besides the water level popped up on the scan. Boat is not overheating. It is tolerable with 2 people and under 1000 of ballast but was hoping to get it fixed before the weekend.

I think what you are dealing with is actually a sea water pressure sensor. It tells the computer that the pump is working and sufficient water is being delivered to cool the engine.
If I am incorrect, please disregard.

Correct. Seems to be the issue right now. Having a PITA finding one though for a 02 mag mpi ski. Might try cleaning it tonight since there was debri in the screen. Have heard it can build up gunk and film around the sensor and have issues. Only one I found was like 120 bucks and it was for the wrong model

Yea its kind of my go to but I've gotten wrong part numbers off of it a few times. Went through my manual last night and couldn't find it either lol. Gonna pull it in a few hours and see if its marked in any way